Juvenile sex ratio in relation to breeding success in Capercaillie Tetrao urogallus and Black Grouse T-tetrix

We examined the relationship between breeding success and juvenile sex rati o in two gamebirds with differing degrees of sexual size dimorphism. We use d breeding success as an indicator of the level of environmental stress the birds had experienced. Capercaillie Tetrao urogallus and Black Grouse T. t etrix wing samples were collected from hunters in northern Sweden during 19 77-86. In both species, annual juvenile sex ratio (percentage of mates) inc reased with annual breeding success, i.e. with reduced environmental stress . We attribute this to the higher energy requirements during growth, vulner ability to environmental stress, and mortality of juvenile males compared w ith females, especially under adverse conditions. Moreover, the relationshi p between breeding success and juvenile sex ratio was stronger in Capercail lie than in Black Grouse. This reflects the higher degree of sexual size di morphism in Capercaillie, and consequent larger mortality difference betwee n juvenile males and females.